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Re: [Xen-devel] [PATCH] VMX virtual-apic: Update the p2m mapping alongsi

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] VMX virtual-apic: Update the p2m mapping alongside the control bit
From: Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Date: Wed, 26 May 2010 09:57:54 +0100
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At 08:22 +0100 on 26 May (1274862163), Keir Fraser wrote:
> On 25/05/2010 15:42, "Tim Deegan" <Tim.Deegan@xxxxxxxxxx> wrote:
> > Otherwise a VM migrated from a system which supports the feature to one
> > which doesn't ends up with a data page where its vlapic should be.
> I don't see that this problem exists, let alone that this patch would fix it
> if it did.

I've seen it happen, if that helps. :) 

> The dummy data page should not be migrated because it has p2m type
> mmio_direct, which will fail the p2m_is_ram() check in mod_l1_entry() when
> dom0 attempts to map the page to save/migrate it.

Ah, so it will; I hadn't seen that.  The full failure mode is actually
on migrating from Xen 3.3 (where mod_l1_entry() didn't have that check)
to a newer Xen on a machine where the HW APIC virt isn't supported.  The
sender sends the contents of the dummy page (incorrectly) and the
receiver doesn't do anything (assuming that there'll be a hole in the
p2m where the APIC should be).

There might be a more correcter thing to do (maybe detect the RAM page
and explicitly free it back to Xen?).  Or, of course, we could finally
get round to explicitly registering all mmio_dm areas...

Sorry for the confusion - I'm trying to tidy up patches from our product
patchqueue and some of them have been around for a while. :|



Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, XenServer Engineering
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

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